Bioelectric signals, including bioelectric potentials and/or bioelectric currents, are monitored and recorded using skin-mounted electrodes to assist in the diagnosis and treatment of many different medical illnesses and conditions.
One example of a bioelectric signal monitored using medical electrodes is the electrical activity of the heart recorded in the form of an electrocardiogram (ECG). The bioelectric signal activity is typically displayed as voltage (.DELTA.V.sub.Heart). The signal ultimately displayed is the composite of several different potentials in addition to those generated by the heart muscle (.DELTA.V.sub.Heart +.DELTA.V.sub.noise).
The fidelity of the signal generated using medical electrodes to accurately conform to the monitored bioelectric signal is typically referred to as trace quality. Trace quality can be represented by the following equation: EQU Trace Quality=.DELTA.V.sub.Heart /(.DELTA.V.sub.Heart +.DELTA.V.sub.noise)(Equation 1)
The change in measured potential due to noise (.DELTA.V.sub.noise) is a combination of a variety of different "artifacts" caused by deformation of the skin, static electricity, induced potentials due to alternating current power sources, radio frequency sources, muscle activity, magnetic fields and triboelectric potentials. These factors all reduce the trace quality of signals produced using medical electrodes placed on a patient's skin to monitor bioelectric signals. John G. Webster, "Interference and Motion Artifact in Biopotentials," IEEE Region 6, 1987 Conference, pp. 53-64, discusses these artifacts and some of the approaches used to reduce their effect on trace quality.
Some of the artifacts, such as those caused by static electricity, alternating current electrical power sources, and radio frequency interference can be reduced using a variety of known methods. One method useful for reducing several of the artifacts is hydration of the skin beneath the electrodes. That hydration occurs spontaneously due to the lower evaporative losses of the skin beneath the monitoring electrodes as well as moisture absorbed from the gels or other electrolyte materials used with many electrodes.
Electronic shielding is also useful to reduce triboelectric potentials in addition to those caused by static electricity, alternating current electrical power sources, and radio frequency interference.
Special circuitry and electrode configurations, typically referred to as a driven-right-leg circuit effectively reduces artifact caused by alternating current power sources. The circuitry essentially involves passing a very small amount of electrical energy into the patient that is out of phase with the induced potentials due to alternating current power sources. This circuitry is described by Winter et al. in "Driven-Right-Leg Circuit Design," IEEE Transactions on Biomedical Engineering, Vol. BME-30, No. 1, January 1983.
Although these approaches at reducing artifacts help to improve trace quality by reducing .DELTA.V.sub.noise, they do not address skin impedance which significantly affects the magnitude of skin deformation artifact. Several approaches have been developed to reduce skin impedance and, thereby, also reduce skin deformation artifact. One approach involves abrading the skin in the area in which an electrode is applied. By abrading skin, the stratum corneum layer is reduced or, in some instances, removed. By reducing the thickness of the stratum corneum, the deformation artifact can be reduced.
Although abrasion is helpful, it can result in large welts and/or scabbing, particularly when performed by an inexperienced operator. At the other end of the spectrum, an inexperienced operator may fail to sufficiently abrade the skin in the desired area, thereby minimally reducing the amount of deformation artifact experienced during monitoring.
Additional drawbacks of the abrasion approach include the additional steps required by the operator to abrade the skin, itching and/or stinging of the abraded skin due to the salts contained in the electrolytes used with many electrodes, as well as the costs associated with providing the materials needed to abrade the skin.
Furthermore, if the operator fails to sufficiently reduce skin impedance through abrasion, the electrode must then be removed from the site and discarded, further abrasion of the skin performed, and then a new electrode must be applied. These additional steps all increase the cost of the procedure, the discomfort of the patient and amount of materials to be disposed.
Another approach at reducing deformation artifact includes increasing the surface area contact between the monitoring electrode and the patient. Cost is a significant disadvantage of this approach because increasing the size of the electrodes increases their cost. In addition, depending on the bioelectric signal to be monitored, it may be difficult or impossible to increase the size of the electrodes sufficiently to significantly reduce the deformation artifact.
Yet another approach at reducing deformation artifact involves piercing the patient's skin in a plurality of locations beneath a monitoring electrode. One mechanism for accomplishing that piercing is disclosed in EPO Publication No. 0 571 120 A1. Although piercing the patient's skin can help reduce deformation artifact, it can also cause irritation and redness or swelling after the procedure has been completed. In addition, in emergency situations, the operator may forget to pierce the skin of the patient or may fail to adequately pierce the stratum corneum to realize the benefits of this procedure, thereby relying on readings that may not be as accurate as expected.
Another approach at reducing deformation artifact includes providing additional chloride ions (Cl.sup.-) between the skin and electrode to increase conductivity through the stratum corneum. Drawbacks of this approach include additional irritation caused by the chloride ions, as well as the additional steps needed to prepare the site and the cost of materials and time. When used in combination with abrasion or piercing, the additional chloride ions can exacerbate stinging and irritation.